1. Technical Field
The present invention relates to a detachably connected multi-pack container assembly having barrier properties.
2. Description of Related Art
Multi-pack container assemblies for storing several separate items are known. Individual servings of refrigerated food products such as yogurt and pudding, and many other products can be packaged individually in such multi-pack container assemblies. These multi-pack container assemblies typically include several containers that are joined together to form a pack of individually sealed servings. Consumers will know that each container can be separated from the multi-pack so that items in the containers can be used.
Typically, conventional multi-pack container assemblies are made by thermoforming polystyrene. Thermoforming is a widely known process for making containers well known to those skilled in the art. In thermoforming, a sheet of thermoplastic resin having a thickness can be clamped into a clamp frame and heated. An oven or heat station heats the edges as well as the center of the thermoplastic sheet so that when the sheet is subsequently delivered to a forming station, an overall temperature balance has been attained. On being heated, the sheet is conveyed to the forming station where by one of several methods it is forced over a mold. The thermoforming of the polyolefin sheet can be performed by means of different, conventional techniques, for instance, a vacuum can be applied in the mold and/or a counter mold can be used to help form the sheet into a container. The residual heat of the plastic can be removed after forming. After cooling, the end product is removed from the forming station and sent to a trim press where the end product is trimmed from the web.
One disadvantage of polystyrene is that is has poor moisture barrier properties. Thus, while polystyrene works well for refrigerated food items such as yogurt and pudding, polystyrene containers are undesirable for items such as low moisture snack foods that require a containers having moisture barrier properties. Unlike polystyrene, polyolefins, such as polypropylene, have excellent moisture barrier properties.
One drawback of using polyolefin containers such as polypropylene is that polypropylene has a high tear strength. Tear strength measures the tear resistance of a material. Because of the high tear strength of polypropylene, it is commonly used in “living hinges.” A living hinge is a flexible hinge of plastic that is molded as one piece with the rest of the container or package, connecting the sturdy top and bottom sections. The material properties of the polypropylene permit the hinge to be flexed repeatedly over time without cracking or breaking Thus, polypropylene packages are not conducive to detachably connected multi-pack assemblies. Consequently, when it's been desired to provide a polypropylene-based multi-pack container assembly, cardboard is used to bundle several separate containers together. The manufacturing process used to produce cardboard bundled individual polypropylene containers, however, is expensive. Material costs are higher because cardboard must be used in addition to plastics material. Further, assembly costs are higher because separate cardboard handling machinery is needed to bundle the individual containers together. Another disadvantage of cardboard bundled polypropylene containers is that a consumer must tear through the cardboard in order to get a container which separates the containers from one another. Once separated, containers are no longer stored as easily as when bundled.
U.S. Pat. Nos. 5,543,104 and 5,409,127 disclose an injection molded container assembly made of high-density polyethylene (“HDPE”). While HDPE containers do have moisture barrier properties, HDPE lacks oxygen barrier properties.
To enhance the shelf life of a food product contained therein, a plastic food container must have adequate barrier properties to protect the product from the migration of moisture or moisture and oxygen into the container. This is typically accomplished by combining, in a layered arrangement, several polymer sheets, each sheet having distinct barrier properties. The typical goal in constructing such container is to provide in the aggregate a layered sheet container that can be constructed at a minimal cost, and yet provide adequate barrier properties to light, moisture, and oxygen without impacting the taste of the product in the container.
For example, EVOH has been found to be an excellent oxygen barrier that reduces oxygen migration into plastic containers. EVOH has been used successfully in combination with, for example, polyethylene, or polypropylene (PP), where the polypropylene or PP provides the moisture barrier properties for the container. Another benefit of using EVOH in containers for food products is its resistance to the migration of oils and contaminates, either from other sheet layers migrating into the product or from the product leaching into the container walls. For example, when post-consumer reground (PCR) polyolefin resins are used as one of the sheet layers for a container, EVOH has been found to be an effective barrier to prohibit contaminates from the PCR resin from being leached into a food product that is placed in the container. An EVOH layer has also been found to be an effective scalping barrier to prevent the absorption of oil and oil-soluble flavors from packaged food.
It is difficult to foresee how a container having a layered arrangement of polymer sheets can be made by an injection molded process, since such process by its very nature fails to provide a layered arrangement because a liquefied plastic mixture must be injected into a mold, resulting in a mixture with no contiguous oxygen barrier layer. Further, the capital costs associated with injection molded items is relatively expensive. Consequently, a need exists for a method for making a detachably connected multi-pack container wherein such container can provide a manufacturer the option of a container having either moisture barrier properties or moisture and oxygen barrier properties.